Total numbers of elementary events are: 6x6 = 36

Let E be the event of getting the sum of numbers on top is less than 7

The favourable combinations are: (1, 1), (1, 2), (1, 3), (1, 4), (1, 5), (2, 1), (2, 3), (2, 2),

(2, 4), (3, 1), (3, 2), (3, 3), (4, 1), (4, 2), (5, 1)

Then, the number of favourable events = 15

∴ P (sum of numbers on top is less than 7) = P (E) = 15/36 = 5/12